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Research points to source of frog deformities

(AP) - The growing number of deformed frogs in recent
years is caused at least partly by runoff from farming and
ranching, new research indicates.

Nitrogen and phosphorous in the runoff fuel a cycle that results
in a parasitic infection of tadpoles, resulting in loss of legs,
extra legs or other deformities, according to researchers led by
Pieter Johnson of the University of Colorado, Boulder.

Their findings are being published in this week's online edition
of Proceedings of the National Academy of Sciences.

The deformed frogs have been a puzzle for more than a decade,
since a group of Minnesota schoolchildren discovered a pond where
more than half of the leopard frogs had missing or extra limbs.
Suggested causes have ranged from pesticides and increased
ultraviolet radiation to parasitic infection.

While parasite infection is now recognized as a major cause of
such deformities, the environmental factors responsible for
increases in parasite abundance had largely remained a mystery,
Johnson said in a statement.

Here's how the cycle works:

The parasites, called trematodes, have a series of host species.

They grow in snails and become infectious when released by the
snails into ponds, where they can infect frog tadpoles, forming
cysts in the developing limbs. Water birds eat the frogs and then
excrete the parasites back into the ecosystem where they can infect
the snails, he explained.

The increasing amount of runoff is fueling a boom in algae
growth, the snails eat the algae and also undergo a population
explosion, increasing the breeding places for the trematodes.

To test the idea, the researchers built 36 artificial ponds in
central Wisconsin and introduced snails. Ponds with added runoff
had a 50 percent increase in the snail population compared with
those that did not have the extra nutrients.